This invention is concerned with the preparation of novel branched block copolymers wherein the individual branches may be independently selected. A number of processes have been disclosed in the prior art for the preparation of polymers, particularly block copolymers, and especially for polymers having a non-linear configuration. These include, for example, the process described in U.S. Pat. No. 3,281,383 wherein polyfunctional coupling agents are suggested for the preparation of branched polymers. The process is limited, however, in that the branches are all substantially identical. This place a structural limitation on the variety of products which can be prepared in that the coupled product is determined by the physical limits and properties of the sum of the individual branches. Up to the present time, no process has been published for the preparation of branched polymers in which the individual branches may vary other than in a statistical manner.
In many end uses it would be desirable to provide polymers in which individual portions of the polymeric molecule could be varied at will to provide additional scope in the set of physical properties of the total polymer. For example, it may be desirable for certain end uses to build in extra flexibility, tensile strength, or processability, all of which heretofore have been limited as described above.
A number of items in the prior art have disclosed multifunctional lithium-based initiators which result in linear or branched polymers. The most pertinent of these relative to the present invention are shown in U.S. Pat. No. 3,644,322 and U.S. Pat. 3,787,510. These patents disclose the preparation of multifunctional initiators by forming a monolithium living polymer, and coupling this polymer with a polyvinyl aromatic compound such as divinylbenzene, resulting in the formation of a oligomer having no more than 15 monomeric units per lithium ion. Consequenly the patents are correct in referring to this product after its coupling with divinylbenzene as "a multifunctional organic lithium ion initiator." This is due to the fact that the molecular weight, even of the coupled product, is so low that it would serve no useful purpose as a polymer even though it may function as a polymerization initiator.
It is an object of the present invention to provide an improved block polymerization process. It is a further object of the invention to provide a process for the production of branched block copolymers. It is a particular object of the invention to provide a process for the production of branched copolymers wherein a high degree of control over the identity of the individual branches may be exercised. Other objects will become apparent during the following detailed description of the invention.